The present invention relates to a developing device for developing an electrostatic latent image on an electrostatic latent image-forming surface into a visible image, and more specifically, to a developing device capable of supplying a developer to an electrostatic latent image-forming surface while held in contact with the image-forming surface.
In dry electrophotography, the developing process is the most critical factor of all influencing the quality of picture. Conventional developing processes include a cascade method, magnetic brush method, and many other systems including modifications of these methods.
Among these systems, the cascade method and magnetic brush method are most frequently used in ammonia process diazo copying machines for business use. These methods are preferred because they facilitate frictional charging of toner and ensure clear copy images.
However, the cascade method is subject to a defect in that peripheral effects produced at the time of developing lower the reproducibility of the solid sections of images. The magnetic brush method, which is free from such a defect, is the most popular system. In the magnetic brush method, however, the working life of the carrier of the developer is relatively short, and the carrier usually must be replaced by new one every time several tens of thousand copies are made.
In order to eliminate the defects of these two methods, the carrier must be improved in shape and material. However, this improvement has not yet been accomplished. In these circumstances, developing systems which use the so-called one-component magnetic developers have been increasingly used. Some of these developing systems use conductive toner, while others employ insulating toner. If one wishes to use ordinary paper as the transfer paper, it is technically advisable to choose the insulating toner for developing, because the aforesaid developing systems involve the following problems. Since it is hard to stably charge the toner, irregular image formation, fogging, variations in concentration and other defects in image or picture will occur. Thus, the use of the insulating, magnetic one-component developer constitutes a hindrance to the manufacture of highly sophisticated copying machines.
Another advantage of the development system using the one-component developer lies in the fact that the toner can develop an electrostatic latent image even though it is not in contact with an electrostatic latent image-forming surface. This advantage is an important factor in the color copying technique which requires superposed development. However, it is difficult to color magnetic toner, since magnetic powder used for the toner is black or brown, and will spoil other colors.
The demand for color copies has greatly increased the need for a developing system which can perform satisfactory color developing using nonmagnetic toner. However, the so-called nonmagnetic, one-component development system is too unsophisticated to be applied to copying machines. According to this system, as is generally known, a thin layer (50 microns or less) or nonmagnetic toner is formed on the surface of a conductive roller, and is opposed to an electrostatic latent image-forming surface such as a photosensitive body, with an electrostatic latent image thereon, with a gap of about 60 microns or less between them. First, as regards this developing system, no reliable means has yet been developed for forming the thin layer of toner. Under the present conditions, the most popular method is one in which the toner is passed between the surface of the conductive roller and, e.g., a rubber blade pressed against the same as disclosed, for example, in U.S. Pat. No. 4,232,628. However, this method has several problems, such as uneven thickness of toner layer, abrasion of blade, etc. Secondly, the thin toner layer formed in this manner must have uniform charges. According to the conventional developing system, moreover, the charging probability of toner is too low to ensure good and stable charging, and the quality of image produced is very poor. Accordingly, tests are being made to improve the nonmagnetic toner in both charging property and filming capability. However, this improvement greatly depends on the material conditions, and is still in an experimental stage. This also holds true of the improvement of the frictional charging capability of the nonmagnetic toner on the surface of the conductive roller.
As "intelligent" copying machines have recently been developed, there has been a demand for switching means for the normal and reverse developing processes. Presently, however, no effective means for that purpose exists.